Achieving Fast Oxygen Reduction on Oxide Electrodes by Creating 3D Multiscale Micro-Nano Structures for Low-Temperature Solid Oxide Fuel Cells.

Fast oxygen reduction reaction (ORR) at the cathode is a key requirement for the realization of low-temperature solid oxide fuel cells (SOFCs). While the design of three-dimensional (3D) structures has emerged as a new and promising approach to improving the electrochemical performance of SOFC cathodes, achieving versatile structures and structural stability is still challenging. In this study, we demonstrate a novel architectural design for a superior cathode with fast ORR activity. By employing a completely new fabrication process comprising a 3D printing technique and pulsed laser deposition (PLD), we design 3D La0.8Sr0.2CoO3-δ (LSC) micro-nano structures with the desired shape. 3D-printed yttria-stabilized ZrO2 (YSZ) microstructures significantly increase the ratio of surface area to volume while maintaining suitable ionic conductivity comparable to that of single-crystalline YSZ substrates. Scanning electron microscopy and energy dispersive X-ray microanalysis reveal the formation of crack- or void-free YSZ microstructures and the uniform deposition of LSC films by PLD on the YSZ microstructures. The 3D LSC micro-nano structures show significantly enhanced oxygen surface exchange coefficients (kchem) extracted from electrical conductivity relaxation (ECR) measurements by up to 3 orders of magnitude relative to the bulk LSC. Furthermore, electrochemical impedance spectroscopy measurements verify the kchem values from ECR and no directional difference in the measured ORR activity depending on the shape of 3D microstructures. The dramatic enhancement of the ORR activity of LSC is attributed to the increased film surface areas resulting from the 3D YSZ microstructures.

Quasi-seamless stitching for large-area micropatterned surfaces enabled by Fourier spectral analysis of moiré patterns.

The main challenge in preparing a flexible mold stamp using roll-to-roll nanoimprint lithography is to simultaneously increase the imprintable area with a minimized perceptible seam. However, the current methods for stitching multiple small molds to fabricate large-area molds and functional surfaces typically rely on the alignment mark, which inevitably produces a clear alignment mark and stitched seam. In this study, we propose a mark-less alignment by the pattern itself method inspired by moiré technique, which uses the Fourier spectral analysis of moiré patterns formed by superposed identical patterns for alignment. This method is capable of fabricating scalable functional surfaces and imprint molds with quasi-seamless and alignment mark-free patterning. By harnessing the rotational invariance property in the Fourier transform, our approach is confirmed to be a simple and efficient method for extracting the rotational and translational offsets in overlapped periodic or nonperiodic patterns with a minimized stitched region, thereby allowing for the large-area and quasi-seamless fabrication of imprinting molds and functional surfaces, such as liquid-repellent film and micro-optical sheets, that surpass the conventional alignment and stitching limits and potentially expand their application in producing large-area metasurfaces.

Microstructured Surfaces for Reducing Chances of Fomite Transmission via Virus-Containing Respiratory Droplets.

Evaporation-induced particle aggregation in drying droplets is of significant importance in the prevention of pathogen transfer due to the possibility of indirect fomite transmission of the infectious virus particles. In this study, particle aggregation was directionally controlled using contact line dynamics (pinned or slipping) and geometrical gradients on microstructured surfaces by the systematic investigation of the evaporation process on sessile droplets and sprayed microdroplets laden with virus-simulant nanoparticles. Using this mechanism, we designed robust particle capture surfaces by significantly inhibiting the contact transfer of particles from fomite surfaces. For the proof-of-concept, interconnected hexagonal and inverted pyramidal microwall were fabricated using ultraviolet-based nanoimprint lithography, which is considered to be a promising scalable manufacturing process. We demonstrated the potentials of an engineered microcavity surface to limit the contact transfer of particle aggregates deposited with the evaporation of microdroplets by 93% for hexagonal microwall and by 96% for inverted pyramidal microwall. The particle capture potential of the interconnected microstructures was also investigated using biological particles, including adenoviruses and lung-derived extracellular vesicles. The findings indicate that the proposed microstructured surfaces can reduce the indirect fomite transmission of highly infectious agents, including norovirus, rotavirus, or SARS-CoV-2, via respiratory droplets.

Octave-spanning mid-infrared femtosecond OPA in a ZnGeP2 pumped by a 2.4†µm Cr:ZnSe chirped-pulse amplifier.

We report on the highly efficient, octave-spanning mid-infrared (mid-IR) optical parametric amplification (OPA) in a ZnGeP2 (ZGP) crystal, pumped by a 1 kHz, 2.4 µm, 250 fs Cr:ZnSe chirped-pulse amplifier. The full spectral coverage of 3-10 µm with the amplified signal and idler beams is demonstrated. The signal beam in the range of ∼3 - 5 µm is produced by either white light generation (WLG) in YAG or optical parametric generation (OPG) in ZGP using the common 2.4 µm pump laser. We demonstrate the pump to signal and idler combined conversion efficiency of 23% and the pulse energy of up to 130 µJ with ∼2 µJ OPG seeding, while we obtain the efficiency of 10% and the pulse energy of 55 µJ with ∼0.2 µJ WLG seeding. The OPA output energy is limited by the available pump pulse energy (0.55 mJ at ZGP crystal) and therefore further energy scaling is feasible with multi-stage OPA and higher pump pulse energy. The autocorrelation measurements based on random quasi-phase matching show that the signal pulse durations are ∼318 fs and ∼330 fs with WLG and OPG seeding, respectively. In addition, we show the spectrally filtered 30 µJ OPA output at 4.15 µm suitable for seeding a Fe:ZnSe amplifier. Our ultrabroadband femtosecond mid-IR source is attractive for various applications, such as strong-field interactions, dielectric laser electron acceleration, molecular spectroscopy, and medical surgery.

Ultrafast fluorescent decay induced by metal-mediated dipole-dipole interaction in two-dimensional molecular aggregates.

Two-dimensional molecular aggregate (2DMA), a thin sheet of strongly interacting dipole molecules self-assembled at close distance on an ordered lattice, is a fascinating fluorescent material. It is distinctively different from the conventional (single or colloidal) dye molecules and quantum dots. In this paper, we verify that when a 2DMA is placed at a nanometric distance from a metallic substrate, the strong and coherent interaction between the dipoles inside the 2DMA dominates its fluorescent decay at a picosecond timescale. Our streak-camera lifetime measurement and interacting lattice-dipole calculation reveal that the metal-mediated dipole-dipole interaction shortens the fluorescent lifetime to about one-half and increases the energy dissipation rate by 10 times that expected from the noninteracting single-dipole picture. Our finding can enrich our understanding of nanoscale energy transfer in molecular excitonic systems and may designate a unique direction for developing fast and efficient optoelectronic devices.

Probing the lithium ion storage properties of positively and negatively carved silicon.

Here, we report Si pillar and well arrays as tailored electrode materials for advanced Li ion storage devices. The well-ordered and periodic morphologies were formed on a Si electrode thin film via laser interference lithography followed by a dry etch process. Two different patterns of negatively or positively carved Si electrodes exhibited highly improved cycle performance as a consequence of the enlarged surface area and the nanoscale pattern effects. The Si well arrays showed the highest energy density, rate capability, and cycling retention among the prepared Si electrodes. This tailored electrode platform demonstrates that these design principles could be applied to future developments in Si electrodes.

Copper nanofiber-networked cobalt oxide composites for high performance Li-ion batteries.

We prepared a composite electrode structure consisting of copper nanofiber-networked cobalt oxide (CuNFs@CoOx). The copper nanofibers (CuNFs) were fabricated on a substrate with formation of a network structure, which may have potential for improving electron percolation and retarding film deformation during the discharging/charging process over the electroactive cobalt oxide. Compared to bare CoOxthin-film (CoOxTF) electrodes, the CuNFs@CoOxelectrodes exhibited a significant enhancement of rate performance by at least six-fold at an input current density of 3C-rate. Such enhanced Li-ion storage performance may be associated with modified electrode structure at the nanoscale, improved charge transfer, and facile stress relaxation from the embedded CuNF network. Consequently, the CuNFs@CoOxcomposite structure demonstrated here can be used as a promising high-performance electrode for Li-ion batteries.

SnO(2) nanorod-planted graphite: an effective nanostructure configuration for reversible lithium ion storage.

We report a novel architecture of SnO(2) nanorod-planted graphite particles for an efficient Li ion storage material that can be prepared by a simple catalyst-assisted hydrothermal process. Rectangular-shaped SnO(2) nanorods are highly crystalline with a tetragonal rutile phase and distributed uniformly over the surface of micrometer-sized graphite particles. In addition, the size dimensions of grown SnO(2) nanorods can be controlled by varying the synthesis conditions. The diameter can be engineered to a sub-100 nm range, and the length can be controlled to up to several hundred nanometers. Significantly, the SnO(2) nanorod-planted graphite demonstrates an initial Li ion storage capacity of about 1010 mAh g(-1) during the first cycle. Also, these SnO(2)-graphite composites show high Coulombic efficiency and cycle stability in comparison with SnO(2) nanomaterials that are not combined with graphite. The enhanced electrochemical properties of SnO(2) nanorod-planted graphite, as compared with bare SnO(2) materials, inspire better design of composite materials with effective nanostructural configurations for advanced electrodes in lithium ion batteries.

Patterned array of nanoparticles on substrates via contact printing method with CNTs/AAO stamp.

Patterned arrays of Fe oxide nanoparticles were transferred via contact printing method on a substrate surface using carbon nanotubes embedded in anodic aluminum oxide (CNTs/AAO) as a stamp, in which vertically aligned CNTs in hexagonally patterned array was first fabricated by chemical vapor deposition into the AAO, followed by a partial chemical etching to expose the CNTs from the AAO. Fe precursor inked CNTs stamp was contact-printed on a Pt-coated Si substrate, and after heat treatment at 200 degrees C, patterned array of Fe oxide nanoparticles with ca. 80 nm of diameter and ca. 120 nm of inter-distance between the nanoparticles was consequently obtained.

Size controlled nickel oxide nanoparticles on carbon nanotubes for supercapacitor electrode.

We have synthesized supercapacitor electrodes fashioned of NiO(x)/multiwalled carbon nanotubes (MWNTs), in which the controlled NiO(x) nanoparticles were prepared via a simple colloidal method and supported on the MWNTs. The sizes of the NiO(x) nanoparticles on MWNTs were systematically varied from 4 to 14 nm at a fixed metal loading of 20 wt% by changing sintering temperature in a controlled manner. The maximum specific capacitance of the NiO(x)/MWNTs was measured to be ca. 215 F per unit gram of NiO(x)/MWNTs by cyclic voltammetry (CV) in an aqueous 1 M KOH electrolyte, demonstrating that the specific capacitance of the MWNTs-supported nanosize NiO(x) is strongly dependent on the dispersion and size of the nanoparticles for the supercapacitor performance.